Soybean (Glycine max (L.) Merr.) yields have increased substantially in the past 50 years, but soybean productivity is still often limited by environmental stresses. Because an inverse relationship often exists between seed yield and stress resistance, a two-year study was initiated to examine the responses of two old, low-yielding ('Manchu' and 'Dunfield') and two modern, high-yielding varieties ('Williams' and 'Clark 63') to long-term soil moisture deficit.A significant soil moisture by variety interaction was found for seed yield in both years. On a relative basis, yield was reduced by drought stress more in the modern varieties due to a greater relative decrease in pod number per branch. The higher absolute yields of the modern varieties under both water treatments were due to more seeds per pod. In terms of leaf area index, Manchu was the most tolerant variety to drought stress, showing small reductions in leaf number and area per leaf under drought. The number of mainstem nodes was reduced by soil moisture deficit in each variety due to a decrease in the rate of node production. A linear relationship was found between node formation and air temperature, and low temperatures delayed the time of flower appearance. Drought stress reduced the number of days to maturity, plant height, internode length, and the seed-filling period.Soil moisture deficit decreased leaflet water potentials in both years. Leaflet water potentials were always lower in Dunfield than in Williams and Clark 63, but were only lower in Manchu in a few instances. Dunfield had lower leaflet osmotic potentials than the modern varieties in the afternoon. In contrast, Manchu had higher osmotic potentials at each sampling period. Leaflet turgor potentials were similar in Clark 63, Williams, and Dunfield throughout the day, whereas those of Manchu were always lower. At most sampling times there was a significant soil moisture by variety interaction for stomatal resistance, with the older varieties having a greater difference between stressed and irrigated plots. Based on the data taken from this experiment, it was concluded that some older varieties of soybean are more drought tolerant than their modern counterparts even though their absolute yields may be lower under stressed and nonstressed conditions.